The present invention pertains to the radio frequency art and, more particularly, to a method for matching the output of a radio transmitter to an antenna.
The prior art has developed numerous methods for the tuning of a radio frequency antenna to the output impedance of the transmitter, as this will assure maximum transmitted power. For fixed position, single frequency installations it is often possible to manually tune to the antenna only once without need for further concern. However, for applications wherein a transmitter is operable on any one of several channel frequencies and/or the antenna is subjected to varying conditions, such as in the case of marine applications, the maintenance of proper antenna tuning becomes very difficult.
One approach to a tunable antenna coupling circuit is given in U.S. Pat. No. 3,906,405, which issued Sept. 16, 1975 and is assigned to the same assignee as the instant application. In this approach, a series of inductors and capacitors are arranged between the transmitter and the antenna with relays operable to switch these components into or out of the circuit. The antenna is then manually tuned to the transmitter for each channel, with the optimum circuit configuration of the tuner being programmed via a diode matrix. Upon the transmitter being tuned to a given channel, the diodes activate the desired relays, thereby forming the proper matching circuit.
While the above described prior art circuit constituted a significant advance in the antenna tuning art, it suffers from numerous advantages. Firstly, that system requires a manual set up of the antenna. Also, it does not automatically account for changes in the impedance of the antenna as may be caused by, for example, by spraying salt water in marine installations.
Other attempts have been made to provide the semiautomatic tuning of an antenna. In one approach, manual tuning is accomplished to within a range, at which time a motor drives a variable capacitance to accomplish proper transmitter matching. Since the overall tuning range of this system is very small, the system provides limited utility.